Inherently tacky, elastomeric microspheres having a solid or hollow morphology are known in the art to be useful in repositionable pressure-sensitive adhesive applications. As used herein, the term "repositionable" refers to the ability to be repeatedly adhered to and removed from a substrate without substantial loss of adhesion capability. Microsphere-based adhesives are thought to perform well in such applications at least in part due to their "self-cleaning" character, wherein substrate contaminants tend to be pushed aside and trapped between the microspheres as the adhesive is applied. Upon removal, the adhesive can then still present a relatively uncontaminated surface for reapplication to the substrate.
Most water based pressure-sensitive adhesive microspheres are stabilized electrostatically. Thus, most known microspheres can undergo coagulation caused by alkali, alkali salts, polyelectrolytes, and repeated cycles of freezing and thawing. Numerous references concern the preparation and/or use of inherently tacky, elastomeric acrylate polymeric microspheres. Silver (U.S. Pat. No. 3,691,140) discloses such microspheres that are prepared by aqueous suspension polymerization of alkyl acrylate monomers and ionic comonomers, e.g., sodium methacrylate, in the presence of an emulsifier, preferably an anionic emulsifier. The use of a water soluble, substantially oil-insoluble ionic comonomer is critical to preventing coagulation or agglomeration of the microspheres.
Baker et al. (U.S. Pat. No. 4,166,152) discloses solid, inherently tacky (meth)acrylate microspheres, which are prepared from non-ionic alkyl (meth)acrylate monomer(s) in the presence of both an emulsifier and an ionic suspension stabilizer having an interfacial tension sufficient to prevent microsphere agglomeration.
Kinoshita (U.S. Pat. Nos. 4,645,783 and 4,656,218) disclose a "repeatedly usable and releasable sheet" coated with an aqueous suspension of microspheres obtained by aqueous suspension polymerization of one or more alkyl(meth)acrylate esters, one or more alpha-monoolefin carboxylic acids, and one or more other vinyl monomers. The microspheres are prepared in the presence of a protective colloid that comprises casein as the main ingredient.
Delgado (U.S. Pat. No. 5,045,569)discloses the composition and suspension polymerization methods of preparation of hollow acrylate microspheres for use as repositionable adhesives. These hollow microspheres display lowered adhesive transfer and greater shear strength than similarly formulated pressure sensitive adhesives based on solid acrylate microspheres.
In general, however, most pressure sensitive adhesives are not microsphere based, but are instead continuous elastomeric coatings or films which possess an appropriate balance of viscoelastic properties.
Bohme et al. (U.S. Pat. No. 3,890,282) disclose a non-microsphere based pressure-sensitive adhesive that is prepared by the solution polymerization of (meth)acrylic acid, a functional polyalkylene oxide, and a combination of a water insoluble n-alkyl (meth)acrylate and a vinyl ester. The functional polyalkylene oxide is a condensation product of meth(acrylic) acid and a polyethylene oxide. Following the polymerization, at least 4 percent of the acid is neutralized. The patent teaches that these adhesives, due to the degree of neutralization of the acid, manifest greatly improved stability toward humidity and moisture.
Ray-Chaudhufi et al. (U.S. Pat. No. 3,891,584) describe a thermally and oxidatively stable non-microsphere hot melt adhesive consisting of a graft copolymer of a vinyl monomer and a non-functional water soluble polyalkylene oxide polymer that has a preferred average molecular weight of 6000. Approximately 15 to 25% by weight of a tackifying resin is included in the adhesive. The reference indicates that a minimum average molecular weight of the polyalkylene oxide polymer to achieve water dispersibility is 3000 with a preferred molecular weight of 6000.
Sunakawa et al. (U.S. Pat. No. 4,442,258) disclose a non-microsphere, film-forming water soluble pressure sensitive adhesive that is prepared by solution polymerization of a (meth)acrylate in the presence of a non-functional polyether polyol or polyhydric alcohol with a molecular weight less than 3000. The reference indicates that an addition polymer of the alkyl (meth)acrylate and/or the alkyl (meth)acrylate with the polyether polyol or polyhydric alcohol forms through a chain transfer mechanism. The reference also indicates that the presence of this addition polymer enhances the compatibility of water soluble plasticizers added to the polymer, which ultimately prevents plasticizer migration and oozing.
Thus, a need exists for pressure sensitive adhesive microspheres which are sterically stabilized such that the micro spheres can offer enhanced stability against coagulation caused by alkali, alkali salts, polyelectrolytes and repeated cycles of freezing and thawing.